1. Technical Field
This description generally relates to gangsaws, and more particularly to managing the use of saw blades within gangsaws.
2. Description of the Related Art
In many sawmills, gangsaws are used to saw cants into individual boards for further processing. Such gangsaws typically include many parallel, circular saw blades located at precise intervals within a sawbox and, at the front of the sawbox, two chip heads (e.g., vertical drum chip heads) for removing excess wood from the outside of the cant. Cants may be transported in a straight line through the gangsaw using feed rolls on the upstream and downstream sides of the sawbox or may be driven through the gangsaw along a curved path during a curve sawing process. Alternatively, the cants may be transported in a straight line while the saw blades and chip heads move to create a curved sawing path. Since gangsaws can have more saw blades than are necessary to saw each cant, many gangsaws also allow automatic or manual selection of which saw blades will be used to saw any particular cant.
The gangsaw saw blades are typically organized into single arbor or double arbor configurations. In a single arbor gangsaw, a series of circular saw blades spaced apart by saw guides are mounted on a splined arbor. The saw guides typically ride on a separate shaft and are plumbed to deliver lubrication to the saw blades. Often, the saw guides are situated so that the saw teeth have just left the saw guides before entering the cant. Thus, the thickness accuracy of the boards near the saw guides is relatively good, even when the gangsaw itself is cutting poorly. A double arbor gangsaw is configured similarly, but with two splined arbors instead of one.
In many gangsaws, the number of saw blades on each arbor exceeds the capacity of the lubrication system. The saw blades may therefore be split into banks, each with a separate lubrication supply. A typical gangsaw might, for example, have 22 saw blades separated into two banks of 11 saw blades each.
As a saw blade in a gangsaw is used, it is subjected to a variety of forces that tend to push or pull the saw blade out of alignment with the other saw blades. For example, during curve sawing, the saw blade may be pushed or pulled in one direction or another by the sawn cant. As another example, if the chip heads upstream of the gangsaw are not removing enough wood, a cant may rub against the saw blades at the edges of the cant, pushing them out of alignment. In fact, even if the sawing process is otherwise going smoothly, the saw blade may encounter a particularly hard section of wood and be forced sideways around that section. This tends to happen with greater frequency as the saw blade ages and becomes duller.
When a saw blade is out of alignment or “lays over,” boards sawn by the defective saw blade no longer have their nominal thickness but show uneven geometries indicative of the sawing characteristic of the saw blade. In a single arbor gangsaw, boards sawn by a defective saw blade may show evidence of “wedging.” The thickness of a board showing wedging typically varies across its width, from a nominal thickness at those portions sawn closest to the saw guides to a maximum variation at those portions sawn farthest from the saw guides.
In a double arbor gangsaw, boards sawn by a defective saw blade may show evidence of “mismatch.” Mismatch occurs when a saw blade is out of alignment with a corresponding saw blade on the other arbor. The thickness of a board showing mismatch is typically close to the nominal thickness at those portions sawn closest to the saw guides and shows a maximum variation at those portions sawn where the saw blades of the top and bottom arbors overlap on the board.
When the wedging or mismatch becomes too significant, a gangsaw must be shut down and the defective saw blade found and replaced, leading to significant losses in productivity. In many sawmills, it can also be difficult to identify the defective saw blade, as mismatch and wedging are often detected only after subsequent processing that mixes the boards.
Even slight wedging or mismatch can also lead to losses at a sawmill. In many sawmills, each piece of final lumber is planed according to the maximum allowed wedging or mismatch (such that regardless of the defective saw blades in the gangsaw, the resulting lumber is planar). This planing process further requires that the lumber is sawn such that sufficient excess material is available. If wedging or mismatch defects could be minimized, the amount of fiber removed by planing could be similarly minimized, thereby maximizing the lumber extracted from each log.
It is desirable therefore to have improved feedback systems that can provide early insight into the geometry of boards sawn by a gangsaw.